The familiar form of a tennis racket includes a bow, an assembly including a shaft, a pallet and a grip that will hereinafter be called a handle, and a throat that connects the bow to the handle. The bow is usually a hoop-like element elongated in the direction of the axis of the handle and having holes drilled through it to accept racket strings. The holes in the bow are drilled opposite each other across the bow so that a group of strings each passing through a hole and its matching hole on the other side of the bow will form a rectilinear pattern when the racket is strung.
In stringing a racket one or two pieces of string are passed back and forth to form all the main strings and all of the cross strings. Each though only one or two continuous strings are used, a single traverse of the racket bow is usually referred to as "a string".
A string traversing the bow through a pair of holes is pulled to a predetermined tension. The pairs of holes in the bow through which a traversing string is drawn are normally not coaxial. The holes are not coaxial for a number of reasons among which are that the holes through which strings are drawn where the throat connects to the bow of the racket must be drilled at rather sharp angles in order to intersect the interior of the bow at the appropriate position. Other holes are drilled at angles to avoid weakening the bow by having all holes drilled through the same grain line, and to avoid having intersecting holes at positons where main strings and cross strings are both strung across the bow, as at the corners of the rectilinear pattern.
In stringing a tennis racket the main strings are strung first. The main strings are passed through a pair of holes on opposite sides of the bow and drawn to what is desired to be the predetermined tension by exerting that amount of force on the string. When the string is tensioned it is clamped to retain that tension and the string is then passed through the next pair of holes and the tensioning and clamping process repeated. A particularly desirable manner for tensioning strings is to overstress them and relax them back to the desired tension which reduces variations in the final tension on the strings due to stretching.
The force exerted on a string to tension it within the racket desirably produces exactly that force as the tension on the string. However, at least some of the tensioning force is dissipated in the friction of the string rubbing against the sharp edge or small radius curve at the entry to a hole in the bow. Obviously, holes that intersect the rim at more acute angles cause greater friction against the string and more of the tensioning force is dissipated in overcoming friction and less of it is available to put the desired predetermined tension on the string.
The problem is even more aggravated when placing cross strings in the racket. In addition to the friction losses due to angular holes as described above, there are friction losses because the cross strings are woven through the main strings and a frictional contact is made at each intersection of the cross strings and the main strings. In fact, the friction between the main strings and the cross strings can become so severe that the strings may be weakened by the stringing process.
Among the problems encountered with tennis rackets having different tensions on different strings is that the racket will respond unpredictably when a ball is hit. If different main strings have different tensions, those that have the greatest tension will produce the greatest effect on the ball when it is struck. Also, when a racket is strung with its strings at different tensions the character of the racket changes as it is used. Expert tennis players thus have to "break-in" a newly strung tennis racket by playing with it before it can be used in serious competition. The breaking in causes the different string tensions to equalize somewhat.
In an effort to produce uniform and predictable tension in tennis racket strings many devices and techniques have been employed. One technique, as mentioned above, is to overstress the string and to approach the proper predetermined tension by relaxing the string to the wanted tension rather than tensioning it to that point. However, whether the desired tension is approached by pulling or relaxing the string, friction always absorbs an unknown of the tensioning force. Another technique that has been employed to diminish the influence of friction is to deflect the main strings when the cross strings are being tensioned so that tension is applied to the cross strings without encountering friction between the cross strings and the main strings. Both of these techniques are useful in the device and process of this invention.
As mentioned above, the response to a tennis racket to striking a tennis ball varies with the tension of the strings and with the difference in tension among the strings. The position on the racket strings where the response to striking the ball is most effective is known as the "sweet spot". Striking the ball in the sweet spot not only gives a desirable and predictable response and good control but it also creates a good subjective feel. It is, accordingly, desirable to enlarge the sweet spot. Recently developed tennis rackets have enlarged the sweet spot by making a larger bow for the racket. Longer strings create a larger sweet spot. In rackets strung with the strings passing through angular holes, the effective length of the racket string is the distance across the inside of the bow between each pair of holes.